If it had non stainless spokes, rust may have expanded the threads not only binding them but stressing the nipple radially. Your added stress of trying to turn them finished them off. Even with stainless spokes, nipples sometimes get bound to the spoke and trying to turn them crumbles the flats.
3 suggestions which will help on borderline cases.
1- use a 3 or 4 corner spoke wrench when working on tight or old wheels. It isn't necessary with good wheels but helps with ones like what you work on.
2- add a drop of penetrating oil to each nipple where the spoke emerges, then spin the wheel to drive it in. Set the wheel aside a few hours for it to work it's magic.
3- cut down an old broom handle, or broken hockey stick if you're Canadian, to about 18" or half a meter. Wedge it between the rim and hub, so that you can use it to push the rim across a bit and take some tension off the spoke you're turning. Don't overdo it, just lighten the load a bit. This will greatly reduce the torque needed to turn the nipples.
__________________
FB
Chain-L site
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